The long-term objective of research in this laboratory is to determine the causes for the increased vulnerability of the aging brain to hypoxia, anoxia, and ischemia. As part of this objective, the hypothesis for study in the current proposal is that aging diminishes the ability of brain tissue to buffer intracellular pH changes. The specific aims of this proposal will be (1) to determine whether aging diminishes the ability of brain tissue to extrude add equivalents via Na+-H+ exchange, and (2) to examine the extent to which HCO3--dependent (Cl -dependent) exchange (transport) contributes to regulation of intracellular pH in brain tissue. Experiments based on these aims will be carried out in hippocampal slices, which are highly vulnerable to anoxia, hypoxia, or ischemia, are not complicated by the presence of a functional cerebrovasculature, and are easily prepared. Slices will be produced from the brains of rats aged 6, 16, and 26 months. Intracellular pH will be determined in slices by monitoring the fluorescence of the acetoxymethylester form of the pH- sensitive dye SNARF-1. Extracellular pH will be measured with H+- selective microelectrodes. The presence and function of Na+-H+ exchange and HCO3 -dependent (Cl -dependent) pH regulatory mechanisms, and the alteration of these mechanisms by aging, will be examined by removal of Na+, HCO3, or Cl from the extracellular space, by the addition of pharmacological blockers of pH regulation (for example, amiloride and DIDS), and by the presentation and removal of acid loads. Knowledge obtained from these studies will be essential for understanding how aging alters neuronal function and survival since pH influences many different cellular processes. Also, this work will provide the framework for future studies regarding how age-related changes in intracellular pH regulation modify synaptic activity in the brain and impair recovery of brain tissue from anoxia, hypoxia, and ischemia. Through such studies, a better understanding of how aging modifies the brain's response to strokes or transient ischemic attacks will be achieved.